With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Also, in consideration of global environments, electric cars and hybrid cars have been recently developed and put into practice, so that there is an increasing demand for lithium ion secondary batteries for large size applications having an excellent storage property. Under these circumstances, lithium ion secondary batteries having advantages such as large charge/discharge capacities and a good storage property have been noticed.
Hitherto, as positive electrode (cathode) active substances useful for high energy-type lithium ion secondary batteries exhibiting a 4 V-grade voltage, there are generally known LiMn2O4 having a spinel structure, LiMnO2 having a zigzag layer structure, LiCoO2 and LiNiO2 having a layer rock-salt structure, or the like. Among the secondary batteries using these active substances, lithium ion secondary batteries using LiNiO2 have been noticed because of large charge/discharge capacities thereof. However, the materials tend to be deteriorated in thermal stability and charge/discharge cycle durability upon charging, and it has been therefore required to further improve properties thereof.
Specifically, when lithium ions are released from LiNiO2, the crystal structure of LiNiO2 suffers from Jahn-Teller distortion since Ni3+ is converted into Ni4+. When the amount of Li released reaches 0.45, the crystal structure of such a lithium-released region of LiNiO2 is transformed from hexagonal system into monoclinic system, and a further release of lithium therefrom causes transformation of the crystal structure from monoclinic system into hexagonal system. Therefore, when the charge/discharge reaction is repeated, the crystal structure of LiNiO2 tends to become unstable, so that the resulting secondary battery tends to suffer from poor cycle characteristics or occurrence of undesired reaction between LiNiO2 and an electrolyte solution owing to release of oxygen therefrom, resulting in deterioration in thermal stability and storage property of the resulting battery. To solve these problems, there have been made studies on materials formed by adding Co and Al to a part of Ni of LiNiO2. However, these materials have still failed to solve the above-described problems. Therefore, it has still been required to provide a Li—Ni composite oxide having a more stabilized crystal structure.
One of factors causing deterioration of characteristics of the positive electrode active substances resides in that surplus lithium is likely to be present on the surface of the particles upon synthesis thereof. A large amount of the surplus lithium present on the surface of the particles tends to cause undesirable gelation of an electrode material when forming an electrode therefrom. In addition, if the surplus lithium is subjected to carbonation, generation of a carbon dioxide gas tends to be undesirably caused owing to reactions within the cell when stored in high-temperature conditions. In order to obtain the Li—Ni composite oxide comprising a less amount of the surplus lithium components, it is necessary that the surplus lithium is removed by subjecting the Li—Ni composite oxide to water-washing treatment. However, since the pH value of the washing solution tends to be increased upon the water-washing treatment, if any amphoteric metal such as Al is present in the form of a solid solution in the Li—Ni composite oxide, the amphoteric metal tends to be eluted out therefrom. On the other hand, when the content of the amphoteric metal in the Li—Ni composite oxide is excessively small, the resulting electrode material tends to be deteriorated in battery characteristics such as cycle characteristics.
More specifically, it has been required to provide Li—Ni composite oxide as a positive electrode active substance for a non-aqueous electrolyte secondary battery which is capable of comprising a less amount of surplus lithium and exhibiting good cycle characteristics.
Hitherto, in order to improve secondary battery characteristics, there are known various techniques for improving cycle characteristics, a storage property and a thermal stability thereof (Patent Documents 1 to 13).